In order to secure passenger safety upon collision and to improve fuel efficiency by reducing the weight of automotive bodies, high-strength steel sheets having a tensile strength (TS) of 780 MPa or more, and reduced in thickness, have been increasingly applied to automobile structural members. Further, in recent years, examination has been made of applications of ultra-high-strength steel sheets with 980 MPa and 1180 MPa grade TS.
In general, however, strengthening of steel sheets leads to deterioration in formability. It is thus difficult to achieve both increased strength and excellent formability. Therefore, it is desirable to develop steel sheets with increased strength and excellent formability.
In addition, strengthening of steel sheets and reducing the thickness significantly deteriorates the shape fixability of the steel sheets. To address this problem, a press mold design is widely used that takes into consideration the amount of geometric change after release from the press mold as predicted at the time of press forming.
However, the amount of geometric change is predicted on the basis of TS, and accordingly increased variation in TS of steel sheets results in the predicted value of geometric change deviating more markedly from the amount of actual geometric change, inducing malformation. Such steel sheets suffering malformation require adjustments after subjection to press forming, such as sheet metal working on individual steel sheets, significantly decreasing mass production efficiency. Accordingly, there is a demand for minimizing variation in TS of steel sheets.
To meet this demand, for example, JP2004218025A (PTL 1) describes a high-strength steel sheet with excellent workability and shape fixability comprising: a chemical composition containing, in mass %, C: 0.06% or more and 0.60% or less, Si+Al: 0.5% or more and 3.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0.15% or less, and S: 0.02% or less; and a microstructure that contains tempered martensite: 15% or more by area to the entire microstructure, ferrite: 5% or more and 60% or less by area to the entire microstructure, and retained austenite: 5% or more by volume to the entire microstructure, and that may contain bainite and/or martensite, wherein a ratio of the retained austenite transforming to martensite upon application of a 2% strain is 20% to 50%.
JP2011195956A (PTL 2) describes a high-strength thin steel sheet with excellent elongation and hole expansion formability, comprising: a chemical composition containing, in mass %, C: 0.05% or more and 0.35% or less, Si: 0.05% or more and 2.0% or less, Mn: 0.8% or more and 3.0% or less, P: 0.0010% or more and 0.1000% or less, S: 0.0005% or more and 0.0500% or less, and Al: 0.01% or more and 2.00% or less, and the balance consisting of iron and incidental impurities; and a metallographic structure that includes a dominant phase of ferrite, bainite, or tempered martensite, and retained austenite in an amount of 3% or more and 30% or less, wherein at a phase interface at which the austenite comes in contact with ferrite, bainite, and martensite, austenite grains that satisfy Cgb/Cgc >1.3 are present in an amount of 50% or more, where Cgc is a central carbon concentration and Cgb is a carbon concentration at grain boundaries of austenite grains.
JP201090475A (PTL 3) describes “a high-strength steel sheet comprising a chemical composition containing, in mass %, C: more than 0.17% and 0.73% or less, Si: 3.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0.1% or less, S: 0.07% or less, Al: 3.0% or less, and N: 0.010% or less, where Si+Al is 0.7% or more, and the balance consisting of Fe and incidental impurities; and a microstructure that contains martensite: 10% or more and 90% or less by area to the entire steel sheet microstructure, retained austenite content: 5% or more and 50% or less, and bainitic ferrite in upper bainite: 5% or more by area to the entire steel sheet microstructure, wherein the steel sheet satisfies conditions that 25% or more of the martensite is tempered martensite, a total of the area ratio of the martensite to the entire steel sheet microstructure, the retained austenite content, and the area ratio of the bainitic ferrite in upper bainite to the entire steel sheet microstructure is 65% or more, and an area ratio of polygonal ferrite to the entire steel sheet microstructure is 10% or less, and wherein the steel sheet has a mean carbon concentration of 0.70% or more in the retained austenite and has a tensile strength (TS) of 980 MPa or more.
JP2008174802A (PTL 4) describes a high-strength cold-rolled steel sheet with a high yield ratio and having a tensile strength of 980 MPa or more, the steel sheet comprising, on average, a chemical composition that contains, by mass %, C: more than 0.06% and 0.24% or less, Si: 0.3% or less, Mn: 0.5% or more and 2.0% or less, P 0.06% or less, S: 0.005% or less, Al: 0.06% or less, N 0.006% or less, Mo: 0.05% or more and 0.50% or less, Ti: 0.03% or more and 0.2% or less, and V: more than 0.15% and 1.2% or less, and the balance consisting of Fe and incidental impurities, wherein the contents of C, Ti, Mo, and V satisfy 0.8≤(C/12)/{(Ti/48)+(Mo/96)+(V/51)}≤1.5, and wherein an area ratio of ferrite phase is 95% or more, and carbides containing Ti, Mo, and V with a mean grain size of less than 10 nm are diffused and precipitated, where Ti, Mo, and V contents represented by atomic percentage satisfy V/(Ti+Mo+V)≥0.3.
JP2010275627A (PTL 5) describes a high-strength steel sheet with excellent workability comprising a chemical composition containing, in mass %, C: 0.05% or more and 0.30% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.5% or more and 3.5% or less, P: 0.003% or more and 0.100%, S: 0.02% or less, and Al: 0.010% to 1.500%, where Si+Al: 0.5% to 3.0%, and the balance consisting of Fe and incidental impurities; and a metallic structure that contains, by area, ferrite: 20% or more, tempered martensite: 10% or more and 60% or less, and martensite: 0% to 10%, and that contains, by volume, retained austenite: 3% to 10%, where a ratio m/f of a Vickers hardness (m) of the tempered martensite to a Vickers hardness (f) of the ferrite is 3.0 or less.
JP3231204B (PTL 6) describes a steel sheet with a multi-phase excellent in fatigue properties, the steel sheet comprising a chemical composition containing, in mass %, C: 0.03% or more and 0.15% or less, Si: 0.3% or more and 1.5% or less, Mn: 0.1% or more and 2.0% or less, P: 0.1% or more, Al: 0.005% or more and 0.1% or less, and S: 0.005% or less, and the balance consisting of Fe and incidental impurities; and a tri-phase structure that contains hard, bainite and martensite phases in ferrite phase, wherein the ferrite has a grain size of 4 μm or more and 15 μm or less and a Vickers hardness (Hv) of 140 or more and 180 or less, the bainite has a grain size of 6 μm or less and a Vickers hardness (Hv) of 250 or more and 400 or less, and the martensite has a grain size of 6 μm or less and a Vickers hardness (Hv) of 400 or more and 700 or less, and wherein a volume fraction of the entire hard phase is 5% or more and 40% or less, and the entire hard phase has a mean free path of 20 μm or less.